Crystal structure of a 1:1 salt of 4-aminobenzoic acid (vitamin B10) with pyrazinoic acid

The paper reports the crystal structure of novel salt of 4-aminobenzoic acid (Vitamin B10) with pyrazinoic acid.


Chemical context
4-Aminobenzoic acid (PABA) is known as vitamin B 10 and is involved in the production of folic acid in bacteria (Chang & Hu, 1996;Akberova, 2002). It is used as an antibacterial (Richards et al., 1995), anti-inflammatory (Flindt-Hansen & Ebbesen, 1991), antioxidant (Sirota et al., 2017;Galbinur et al., 2009), anticoagulant (Stroeva et al., 1999;Drozd et al., 2000), or dermatologic agent (Rothman & Henningsen, 1947;Xavier et al., 2006;Hanson et al., 2006). Moreover, it is a building block used in the design of drug candidates and is frequently found as a structural moiety in drugs (Kluczyk et al., 2002). PABA has been the subject of many scientific investigations, due not only to its pharmaceutical and biological properties, but also its ability to form various multi-component solid forms. PABA is a simple organic molecule with two functional groups: amine and carboxyl. This makes it unique in its ability to form various hydrogen-bonded network structures (Athimoolam & Natarajan, 2007). Among all the multi-component crystals of PABA known to date, co-crystals and salts of PABA are especially numerous. ISSN 2056-9890 Today, the formation of either salts or co-crystals of APIs is one of the promising strategies to modify the solid-state properties of pharmaceutical compounds, such as solubility, bioavailability, stability, etc. (Shevchenko et al., 2012;Perumalla & Sun, 2013;Manin et al., 2018). The main difference between a salt and a co-crystal is in the position of a proton. A salt is formed if a proton is transferred from an acid to a base (Aakerö y et al., 2007). Childs et al. (2007) and Cruz-Cabeza (2012) have noticed a linear correlation between ÁpK a [pK a (base) -pK a (acid)] of the starting compounds and the probability of the formation of either a salt or a co-crystal. It is assumed that a salt is expected to be formed if ÁpK a > 3 (Childs et al., 2007) or ÁpK a > 4 (Cruz-Cabeza, 2012), whereas a co-crystal forms when ÁpK a < 0 (Childs et al., 2007) or ÁpK a < À1 (Cruz-Cabeza, 2012). In the intermediate ÁpK a range, the nature of multi-component crystal is difficult to predict -a so called 'salt-co-crystal continuum' (Childs et al., 2007;Hathwar et al., 2010). Several examples have been documented where both a salt and a co-crystal could be formed by the same components from the same solutions under different crystallization conditions (Fu et al., 2016;Losev & Boldyreva, 2018a,b). A co-crystal can also be converted into a salt in the solid state upon temperature variations (Grobelny et al., 2011).
The present study reports the synthesis and crystallization of a novel salt of 4-aminobenzoic acid with pyrazinoic acid (pyrazine-2-carboxylic acid, POA), [PABA-POA], which was characterized using single crystal and powder X-ray diffraction (SCXRD, PXRD) and different scanning calorimetry (DSC).
2. Elucidation of the multi-component crystal nature 4-Aminobenzoic acid is an ampholyte molecule with basic (-NH 2 ) and acidic (-COOH) functional groups, and its pK a values are 2.46 and 4.62 (Avdeef, 2017) respectively. Pyrazinoic acid is a weak acid with a pK a of 2.9 (Zhang et al., 1999). According to the ÁpK a of PABA and POA, the two-component crystal is within the range of the 'salt-co-crystal continuum'. Both a salt and a co-crystal can be expected to crystallize.
The crystal structure of the title compound was solved and refined at 150 K (Ia) and 293 K (Ib). The nature of the crystal form (salt/co-crystal) was identified from the structural characteristics, namely the C-N bond length of PABA and the C-O bond lengths of the carboxylic/carboxylate groups of PABA and POA at both temperatures to eliminate the possibility of salt-co-crystal transition. In a neutral pure PABA molecule, the length of the C-N bond between the N atom of the amine group and the C atom of the benzene ring is ca 1.37-1.4 Å . In the title compound, the protonation of the PABA amine group results in a significantly longer C-N bond [1.455 (5) Å at 150 K and 1.467 (3) Å at 293 K]. To define the deprotonation site, the C-O bond lengths of both PABA and POA were compared. In a neutral carboxylic group, C-O is longer than C O by 0.08 Å , or more. Deprotonation of a -COOH group leads to a decrease in this difference to 0.03 Å or less (Childs et al., 2007;Chen et al., 2012). In the title compound, the difference d(C-O) is 0.104 (6) or 0.102 (8) Å for PABA and 0.007 (6) or 0.012 (6) Å for POA at 150 K and 293 K, respectively, indicating deprotonation of the POA -COOH group and the formation of a salt.

Structural commentary
The title compound crystallizes in the monoclinic noncentrosymmetric space group Pc with one molecule of each component per asymmetric unit (Fig. 1) The asymmetric unit of the title compound at 150 K, with displacement ellipsoids drawn at the 50% probability level for non-H atoms. H atoms are shown as spheres of arbitrary radii.

Thermal analysis
The thermal behavior of the title compound was investigated by DSC techniques. The DSC curve [PABA+POA] is shown in Fig. 3. For a comparison, the DSC curves of the starting compounds are also plotted. PABA and POA show single endothermic peaks at 188.5 and 224.8 C, respectively.
[PABA+POA] exhibits a sharp endothermic peak at 166.1 C. The melting temperature of the salt is ca 20 and 60 C lower than that of the starting compounds, suggesting the formation of a new crystalline phase. A single endothermic peak for the salt indicates that the solid state is homogeneous, and also suggests that there is no solvent in the crystal.   Table 1 Hydrogen-bond geometry (Å , ) for Ia.

Figure 4
Comparison of the experimental PXRD patterns of [PABA+POA] prepared by liquid-assisted grinding (blue) of PABA (black) and POA (red) and calculated (green) using single-crystal X-ray diffraction data. homosynthon between PABA molecules or PABA and conformer molecules with carboxylic functional group.

Synthesis and crystallization
A commercial sample of PABA (Merck, 99%) was co-crystallized with POA (Acros organics, 99%) by either liquidassisted grinding, or by slow evaporation from solution under ambient conditions. Single crystals of [PABA+POA] were grown at room temperature by slow evaporation of a waterethanol (1:1 v/v) solution in a 1:1 stoichiometric ratio. The powder sample of the title compound for DSC analysis was obtained by liquid-assisted grinding of the physical mixture in the presence of ethanol using a planetary micro mill. The ground material was characterized using PXRD to verify the formation of a new phase by comparing the diffraction pattern with the powder pattern calculated based on the single crystal X-ray diffraction data obtained in this work (Fig. 4).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3

sup-2
Acta Cryst. (2018). E74, 1923-1927 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.